This invention relates to industrial painting lines, and, more particularly, to the ventilation of such lines to collect and dispose of organic compounds and particulates.
In one type of industrial painting line, objects that have previously been formed or processed to shape are placed onto a conveyer and passed through stationary painting booths and a curing facility. The painting booths typically include an automatic painting booth in which a robot applies a coat of paint, followed by a touch up booth where a worker applies paint to areas such as the interior of channels which might be missed by the automatic applicator. It is common practice to have two or more automatic booths and two or more touch up booths, so that the different sides and faces of the object can be painted without turning the object, or so that multiple coats of paint can be applied. After painting, the conveyer moves the objects into a flash off tunnel, where a portion of the volatile organic compounds (VOCs) in the paint are vaporized, and then to the curing oven where the paint is cured to a hard, dry state.
Many different types of paints are available, including organic-based, water-based, and mixed types. (As used herein, the term "paint" is to be interpreted broadly to include all coatings that contain particulates and organic compounds, and without a narrow meaning as may be found in some fields.) Experience has shown that for many painting applications, such as the painting of aluminum, a relatively high organic content of the paint results in the best quality of the painted product. As an example, an organic-based paint producing a high-quality finish on aluminum may contain about 5.5 pounds of volatile organic compounds per gallon, while paints producing lower quality finishes on aluminum may contain 1.5 pounds of organics per gallon or less.
Unfortunately, the high-organic content paints also have the greatest potential for atmospheric pollution. Such paints include both particulate and vaporizable organic compounds, which can escape to the atmosphere during the painting operation. When the painted object passes through the flash off tunnel and the curing oven, organic vapor is evolved. Although the applicator heads in the automatic and touch up booths are configured to deposit a large fraction of the paint onto the objects being painted, inevitably some misses. Both particulate and organic vapor are thereby introduced into the atmosphere.
In much of the United States and in many foreign countries, it is common practice simply to permit the release of the vaporized organic compounds to the environment. Other areas have placed strict limits on the amounts of allowable organics that may be released. For example, in Riverside County of Southern California under rules of the South Coast Air Quality Management District (SCAQMD), the present limit on the release of organics to the atmosphere is 68 pounds per day. A painting line operator may use the high-organic content paint having 5.5 pounds of volatile organic compounds per gallon and producing a high-quality finished product, but is limited to an organic compound loss per day equivalent to that found in only 12.4 gallons of paint. If the operator chooses to use the paint having 1.5 pounds of volatile organic compounds per gallon, the result is a lower-quality finished product, but the operator is permitted an organic compound loss per day equivalent to that found in 45.3 gallons of paint. The environmental laws therefore place the operator in the position of choosing a lower volume, high quality operation, or a higher volume, lower quality operation. It is expected that in the future such organic emissions limitations will become more widely legislated and more strict, throughout the United States and other parts of the world. Moreover, painting line operators may be required to adopt the best available technology, regardless of the rules that apply otherwise.
Federal and state occupational health and safety laws also play a major part in painting line design and operation. Persons who work in such facilities must be protected against overexposure to organic vapors with sufficient ventilation of the workplace. Also, the areas where combustibles such as organic vapors are present must maintained well below the lower explosive limit for the organic-containing vapor.
Thus, painting lines must be operated in compliance with environmental laws and the health and safety laws, and in a manner that produces a high-quality product as economically as possible. Industries located in areas that have strict environmental and health and safety laws are placed in a difficult competitive position against those which are not so located, for example, those in many foreign countries.
There have been attempts to provide painting line systems that permit operation in compliance with the laws, and that also allow the use of paints with high levels of volatile organic compounds. However, in most instances these approaches have not been economically realistic for the types of painting systems discussed above, and there remains a need for such paint line systems. The present invention fulfills this need, and further provides related advantages.